This invention relates in general to graphic recorders, and in particular relates to recorders, such as incremental plotters, for use with strip charts.
Prior art graphic recorders of the type described commonly employ rollers having sprocket pins which engage positioning holes formed along the edges of a strip chart, with the rollers being driven so as to move the strip chart along an X-axis while a marking instrument is moved along a Y-axis for recording information on the chart. An example of such a recorder is that described in U.S. Pat. No. 3,673,604 issued June 27, 1972 and assigned to the assignee of the present invention. Heretofore there has been a difficult problem in maintaining precise registry of the strip chart on the rollers, with the result that recording accuracy has not been optimum and chart speed has been limited. Among the problems which arise are that the chart paper can change dimensions as much as seven percent due to changes in temperature and moisture, or both. Another trouble source is that of uneven wear on the positioning holes from engagement by the sprocket pins, with the result that the chart does not maintain proper registry on the rollers where repeated passes are made through the recorder. In addition, improper registry of the chart can result from manufacturing inaccuracy and dimension tolerances of the positioning holes. Motor
A number of devices have been developed in an attempt to correct the foregoing problems. Among these devices are spring-loaded floating sprockets or floating shafts which are arranged so as to tension the paper in the recorder. However, such devices have not been entirely satisfactory for a number of reasons. The devices inherently develope undesirable friction loading on the paper which increases the paper driving power requirement, degrades registration accuracy and limits the maximum paper travel velocity. Where repeated passes are made over the same section of the chart these problems are compounded. In certaian of the devices the floating sprockets are individually spring-loaded rotationally against the paper to create tension. However, such sprockets act as small flywheels so that as high direction reversal rates are encountered the inertia of the sprockets causes the pins to forefully engage and wear the positioning holes.
Another problem associated with previous graphic recorders is that which arises from the use of fan-fold type paper as the strip chart. The alternate ridges and valleys of the creases in this type of paper present an uneven surface under the writing stylus. The ridges in the paper especially interfere with proper writing action. Heretofore graphic recorders have employed various devices to solve this problem, such as the use of a relatively small diameter rod which supports the paper underneath the stylus. However, the use of such a rod introduces additional frictional drag on the paper, and this is objectional from the standpoint of power requirements for driving the paper and further limits the travel speed of the paper.